1. Field of the Invention
The present invention relates to a magnetic head core for a video tape recorder (VTR), a floppy disk drive (FDD) or other devices, and more particularly to such a magnetic head core which has improved overall or total characteristics and which is suitable for large-scale or mass production. The invention is also concerned with a method suitable for producing such a magnetic head core.
2. Discussion of the Prior Art
A magnetic head core for a VTR, FDD or similar device generally has a structure consisting of two or more ferrite blocks that are bonded together so as to form one magnetic path or two magnetic paths. Known magnetic head cores having an information writing/reading function are illustrated in FIGS. 14 and 15, wherein a magnetic gap 6 is defined by and between opposed magnetic-gap defining protrusions 4A, 4B which are formed so as to protrude from mutually abutting surfaces of two ferrite blocks 2A, 2B. The magnetic gap 6 has a suitable amount of spacing therebetween. On both sides of the magnetic gap 6, there are formed two track-width defining cutouts 8, 8 which define the width of the information writing/reading track. Each of these cutouts 8, 8 provides a considerably larger spacing between the two ferrite blocks 2A, 2B, than the spacing of the magnetic gap 6, in the direction perpendicular to the plane of the gap 6. In FIGS. 14 and 15, reference numeral 10 denotes masses of a non-magnetic bonding filler which fills the track-width defining cutouts 8, 8, and thereby bonds the two ferrite blocks 2A, 2B. Usually, the bonding filler 10 is a glass material.
In such a writing/reading magnetic head core constructed as described above, the track-width defining cutouts 8 are formed on both sides of the magnetic-gap defining protrusions 4A, 4B such that the spacing or distance between the two ferrite blocks 2A, 2B increases in the directions away from the opposite ends of the magnetic gap 6, toward the opposite surfaces of the ferrite blocks 2A, 2B which define their width. It is a generally recognized requirement that an angle .alpha. formed by an extension line of the magnetic gap 6 and side surfaces 12 of the protrusions 4A, 4B be relatively small, to enable the magnetic head core to exhibit desired operating characteristics. Described more specifically, where the angle .alpha. is relatively large, the magnetic head core tends to suffer from easy magnetic flux saturation at the opposite ends of the magnetic gap 6 during an information writing operation, and an excessively increased magnetic resistance at the ends of the gap 6 during an information reading operation, and consequent reduction in the information reading efficiency of the head core.
Conventionally, the track-width defining cutouts 8, 8 on both sides of the magnetic-gap defining protrusions 4A, 4B are formed by machining with a grinding wheel. This conventional method does not permit the mass production of the magnetic head core, with the sufficiently small angle .alpha. between the extension line of the magnetic gap 6 and the side surfaces 12, 12 of the protrusions 4A, 4B. Thus, the magnetic head core produced according to the known method is not satisfactory, in terms of information reading/writing efficiency.
To reduce the angle .alpha. indicated above according to the conventional method, the magnetic-gap defining protrusions 4A, 4B should be formed with tapered side surfaces, by using a grinding wheel whose grinding edge has a sufficiently smaller taper angle. This method, however, may result in a considerable amount of variation in the width of the end faces of the protrusions 4A, 4B, i.e., variation in the track width of the magnetic head, which variation arises from positioning error of the grinding wheel, wear of the grinding edge of the wheel or fluctuation in the dimensions of the ferrite blocks 2A, 2B. In view of this low accuracy of the track width, the conventional method for the mass production of the magnetic head uses a grinding wheel whose working angle corresponding to the angle .alpha. is sufficiently large or equal to 90.degree., for forming the magnetic-gap defining protrusions 4A, 4B (cutouts 8 or side surfaces 12). Accordingly, the angle .alpha. between the plane of the magnetic gap 6 and the side surfaces 12 of the protrusions 4A, 4B is necessarily large in the known magnetic head cores, as indicated in FIGS. 14 and 15, whereby the information writing and reading characteristics of the head cores are not practically satisfactory, or required to be further improved.
Keeping pace with an increasing requirement for high-density storage of information per unit area of a storage medium, the medium has a tendency toward reduction in the spacing between the adjacent tracks. The reduced track spacing of the medium leads to an increased crosstalk between the adjacent tracks during an information reading operation of the magnetic head core, namely, a deteriorated off-track operating characteristic of the head core. To minimize the problem of this deteriorated off-track characteristic, there is proposed a magnetic head core as shown in FIG. 16, wherein the width of the end face of one of the two magnetic-gap defining protrusions 4A, 4B is made slightly larger than that of the other protrusion, so that the information writing track width is substantially larger than the information reading track width, as indicated in the figure, due to a so-called "fringing effect" which is provided based on leakage of the magnetic flux. This arrangement reduces the off-track crosstalk, thereby improving the off-tracking operating characteristic of the head core.
In the conventional magnetic head core of the type shown in FIG. 16, too, the angle .alpha. formed between the plane of the magnetic gap 6 and the side surfaces 12 of the protrusions 4A, 4B cannot be made sufficiently small according to the conventional mass production method using a grinding wheel. Hence, the head core does not provide a sufficient fringing effect, and cannot have a sufficiently enlarged effective writing width. In other words, the conventional method does not provide a significant improvement in the off-track operating characteristic of the head core, and does not enable the head core to satisfy the requirement for the high-density storage of information per unit area of a magnetic storage medium.
On the other hand, the provision of a magnetic film such as a film of Sendust for a magnetic gap of the magnetic head core is known to be effective to attain the high-density storage of information relying on a high value of coercivity of a magnetic storage medium. Described more particularly, the magnetic film having a higher saturation magnetic flux density than the ferrite blocks is applied so as to cover the end face of at least one of the two magnetic-gap defining protrusions 4A, 4B of the ferrite blocks, so as to provide a so-called metal-in magnetic gap. A sputtering techinque is known to be more suitable for forming such a magnetic film or films, than vacuum vapor deposition, ion plating, CVD (chemical vapor deposition), plating or other methods, because the sputtering method suffers from a comparatively small amount of fluctuation in the composition of the film(s) and does not considerably limit the magnetic material to be used for the film(s). According to the conventional mass production method, the magnetic film is formed by sputtering so as to cover the entire surface area of each magnetic-gap defining protrusion 4A, 4B, as indicated at 14 in FIG. 17. Since the side surfaces 12 and the end faces of the protrusions 4A, 4B form a relatively large angle (i.e., 90.degree. in this example) therebetween, the magnetic film 14 inevitably has a reduced thickness and insufficient adhesion to the appropriate surface portion at and near the opposite ends of the magnetic gap 6. Consequently, the magnetic film 14 tends to be separated easily from the corner portions of the protrusion 4A, 4B when the ferrite blocks 2A, 2B are bonded together with a bonding filler 10. The magnetic material of the separated magnetic film 14 may disperse, migrate or diffuse into the bonding filler 10, and the slidability of the head core at its sliding surface with respect to a storage medium may be deteriorated by the magnetic material which diffuses to the surface portions of the bonding filler 10. Further, the portion of the magnetic film 14 existing adjacent to the magnetic gap 6 may cause a substantial change in the track width of the head core, which is a serious problem with the magnetic head core.
The magnetic head core of the type described above may pick up signals from the tracks adjacent to the track being read, or suffer from the so-called crosstalk, if the surfaces of the track-width defining cutouts 8, 8 of the two ferrite blocks 2A, 2B have portions which are parallel or almost parallel to the plane of the magnetic gap 6. To avoid this crosstalk, the cutouts 8 are formed with inclined surfaces 16 which are inclined at a suitable angle with respect to the plane of the magnetic gap 6, as indicated in FIGS. 14-17. In the examples of FIGS. 15-17, the inclined surfaces 16 are constituted by curved surfaces which terminate in the curved side surfaces 12 of the protrusions 4A, 4B, such that the surfaces 16, 12 cooperate to form a curvature.